Toothpaste compositions are well characterised in the literature and many compositions are disclosed in patent specifications and other literature. Toothpaste compositions contain a number of specific components for example abrasive agents, fluoride sources, binders, preservatives, humectants, anti plaque agents, colouring agents, water, flavour and other optional ingredients.
Of these components the abrasive agent is required to provide the appropriate cleaning and plaque removal without subjecting the tooth itself to excessive abrasion. Typically a toothpaste composition will contain from about 5% to about 50% preferably up to about 30% by weight of abrasive. Commonly used abrasives are aluminas, calcium carbonates and calcium phosphate. More recently synthetic silicas have been adopted because of their efficient cleaning, compatibility with other ingredients and their physical properties.
An important property of a silica for use in toothpaste formulations is its oil absorption capacity. For silicas of similar particle size, this property directly relates to the thickening effect obtained when adding the silica into a toothpaste formulation, the higher the oil absorption capacity the higher the observed thickening effect. Therefore the higher the oil absorption capacity, the lower the volume of silica which can be incorporated into the toothpaste composition, something which limits the toothpaste formulation. Moreover, the higher the oil absorption, the weaker the structure.
Another important property of a silica for use in toothpaste formulations is its ability to provide the appropriate stain and plaque removal (herein after referred to as the cleaning capacity) without subjecting the tooth itself to excessive abrasion i.e. without damaging dentine or enamel. Normally the cleaning capacity is correlated with the abrasion properties. The concept of preventing stain build-up on a tooth surface must be distinguished from stain removal, which is a considerably more difficult task. A proteinaceous pellicle film forms continuously on teeth and through staining with food and drink can become coloured. If this is allowed to build up with time, the coloured pellicle becomes considerably thicker and more mineralised than the 12-24 hour film and consequently more difficult to remove. Normal dental abrasives are fully capable of controlling 12-24 hour pellicle, but are relatively ineffective at removing old stain.
Another important property of a silica for use in transparent toothpaste formulations is its apparent refractive index. Any transparent toothpaste can be characterised by its refractive index, when incorporating an abrasive material into a transparent toothpaste it is important that this abrasive material remains invisible, i.e. that the clarity of the toothpaste remains the same. This is achieved only if the abrasive material has an apparent refractive index which matches the refractive index of the toothpaste. Now, toothpastes can have refractive indices ranging from 1.430 to 1.470. A refractive index of above 1.445 is generally considered as a high refractive index.
Toothpastes in the form of clear gels are now proposed to consumers, some of those gels being absolutely water white. The method for assessing clarity in this invention involves use of a standard chart consisting of black symbols varying in size on a white background. This is the RIT Alphanumeric Resolution Test Object, RT 4-74, produced by Graphic Arts Research Center, Rochester Institute of Technology. The ability to discern the symbols clearly through a sample of product of standard thickness (1 cm) is measured. The symbols are numbered from -12 to +13. The higher, more positive the number, the greater the clarity. In the present invention a number of 0 or above is considered to be characteristic of a visually clear toothpaste.
In U.S. Pat. No. 5,225,177 is claimed an amorphous silica having a moisture of 10%, a 5% pH of 7, an oil absorption of less than 125 cc/100 g, a refractive index of 1.45. It is further stated that the precipitated silicas according to this document have an RDA value of at least 40, preferably 70 to 120. A detailed description of the method used for measuring the RDA value is provided and under `E. Test Run` it is clear that the RDA value which is given is not the RDA of the silica but the RDA of a toothpaste containing this silica. Moreover on column 11 under `Calculations` it is made clear that the RDA values are given `for a particular paste`. Now, It is not disclosed what is the nature of the toothpaste and, more importantly, the toothpaste silica loading is not disclosed (6% to 35% according to column 5 line 25). The RDA values therefore refer to the abrasion property of an unknown toothpaste containing an unknown amount of a specific amorphous silica and it is not possible, relying on U.S. Pat. No. 5,225,177 to know what is the RDA value of the silica.
Now, the applicant of U.S. Pat. No. 5,225,177 is marketing a product called Zeodent 115 (Average particle size 9.3 .mu.m, refractive index 1.45, oil absorption 110 cc/100 g) which is believed to be the silica disclosed in U.S. Pat. No. 5,225,177. The RDA value of this silica is 97 which is regarded as a low to medium abrasive silica.
Commercially available silicas can be broadly categorised as low abrasion if less than 90 RDA and medium abrasion if between 110-150 RDA. Samples of commercially available toothpaste silicas were submitted to Missouri Analytical Laboratories and the RDA value of the silica was determined with the following results:
______________________________________ SILICA NAME RDA ______________________________________ ZEODENT 113 84 ZEODENT 115 97 TIXOSIL 73 83 SIDENT 9 113 SIDENT 12 91 SORBOSIL AC77 125 SORBOSIL AC35 110 ______________________________________
(NB: Zeodent, Tixosil, Sident and Sorbosil are registered trade marks of Huber, Rhone Poulenc, Degussa and Crosfield respectively).
From the data, it can be seen that even current low abrasion silicas have relatively high RDA values and there is a need for an amorphous silica having a much lower RDA value which, when incorporated into an oral composition, maintains good cleaning characteristics. There is also a need for such an amorphous silica which does not alter the clarity of the toothpaste composition to which it is added.